Question

I’ve been experimenting with Linq to see what it can do – and I’m really loving it so far 🙂

I wrote some queries for an algorithm, but I didn’t get the results I expected… the Enumeration always returned empty:

case #1

List<some_object> next = new List<some_object>();
some_object current = null;

var valid_next_links =
    from candidate in next
    where (current.toTime + TimeSpan.FromMinutes(5) <= candidate.fromTime)
    orderby candidate.fromTime
    select candidate;

current = something;
next = some_list_of_things;

foreach (some_object l in valid_next_links)
{
    //do stuff with l
}

I changed the query declaration to be inline like this, and it worked fine:

case #2

 foreach (some_object l in
      (from candidate in next
       where (current.toTime + TimeSpan.FromMinutes(5) <= candidate.fromTime)
       orderby candidate.fromTime
       select candidate))
 {
  //do stuff with l
 }

Does anybody know why it doesn’t work in case #1 ?
The way I understood it, the query wasn’t evaluated when you declared it, so I don’t see how there is a difference.

Answer 1

Changes to current will be captured, but the query already knows the value of next. Adding extra items to the existing list will make them show up in the query, but changing the value of the variable to refer to a different list entirely won’t have any effect. Basically, if you mentally expand the query from a query expression into a “normal” form, any variable present in a lambda expression will be captured as a variable, but any variable present directly as an argument will be evaluated immediately. That will only capture the reference value of the variable, not the items present in the list, but it still means changing the variable value itself won’t be seen. Your first query expands to:

var valid_next_links = next
      .Where(candidate => (current.toTime + TimeSpan.FromMinutes(5) <= candidate.fromTime))
      .OrderBy(candidate => candidate.fromTime);